The present invention relates to a compact electron gun having a cold microdot electron source.
In general terms, the electron gun according to the invention has a cold microdot electron source, a simplified electronic optics compared with known electron guns, as well as a high voltage anode bombarded by the electrons from the cold source. The source-optic-anode assembly is compact and has a volume of a few cubic centimeters.
This electron gun can be used in a semiconductor laser as an electronic pumping means.
A microdot semiconductor laser or MSL is constituted by a semiconductor source as the active medium and a compact microdot electron gun as the pumping means. This structure can be constituted by a solid semiconductor material, thin film semiconductor materials or a heterostructure.
The use of the electron gun according to the invention makes it possible to improve the performance characteristics of the semiconductor laser and in particular makes it possible to obtain an operation with a weak electronic current, an improvement of the laser efficiency, a limitation of thermal problems, an increase in the life of the pumping means, and a better overall reliability of the laser. These improvements are obtained through modifying and optimizing the compact electron gun according to the invention.
A MSL type laser in particular makes it possible to emit a visible laser light from 0.4 to 0.6 .mu.m, which has numerous applications. Thus, such a laser can be used for the optical recording and reading of information, such as on audio and video compact disks (CD-ROM's--compact disks-read only memories ), WORM's (one write-several reads), erasable memories (of the magnetooptical type or phase change type) or in laser printers.
It can also have other applications such as, for example, bar code readers, laboratory instrumentation, spectroscopy, biomedical instrumentation, pointers, spectacles, display by projection, submarine communications, etc.
The compact electron gun according to the invention can also be used in fields other than that of lasers and in general terms in any device where it is necessary to have electrons accelerated with high voltages (typically 0 to 40 kV), which are focussed and in which everything is assembled in a compact manner (typically a few cm.sup.3). For example, it is possible to use the electron gun according to the invention in a X-ray generator.
For the optical recording and reading of information, the compact laser using the electron gun according to the invention makes it possible to increase the recording density and simplify the optical instrumentation. In laser printers, the compact laser using the electron gun according to the invention permits a better definition of the image and an increase in the printing speed compared with known systems.
An electronic pumping-type compact semiconductor laser using an electron gun with a microdot source is described in FR-A-2 661 566 filed in the name of the applicant. This laser has a laser cavity equipped with a semiconductor heterostructure, which is the laser active medium. This heterostructure forms an anode raised to a high voltage and which is bombarded with high-energy electrons supplied by a compact gun.
Electronic pumping-type lasers are used in cases where the semiconductor materials chosen do not make it possible to produce injection laser diodes and when there is no compact optical pumping source.
The advantages of a MSL compared with injection laser diodes is in particular the separation of the functions the pumping elements and laser cavity.
In injection laser diodes (the only commercially available compact lasers), these basic functions (pumping and cavity) are produced on the semiconductor by appropriate P and N-type electrical dopings of the different epitaxied layers and by ohmic contacts.
The different manufacturing operations for obtaining these diodes make it necessary to perfectly control the technology of producing heterostructures and are generally only possible for certain semiconductors from the family of III-V compounds (of the type GaAlAs). This limits the wavelength range accessible to laser diodes to between 0.6 and 1.5 .mu.m.
In MSL-type lasers, the injection of carriers ( electrons and holes ) which recombine in the active zone of the semiconductor for generating light emission, by definition takes place by an external source (electron gun) with respect to the active semiconductor medium. Consequently there is no need for a P or N-type doping of the different epitaxied layers of the laser. Electrical contacts are also unnecessary.
This greatly simplifies the metallurgy of the active semiconductor medium, where consideration is only necessary of the electrical confinement characteristics (electronic pumping/quantum wells), optical confinement characteristics (light guidance with a mode centred on the active zone) and the wavelength characteristics.
This advantage makes it possible to use in MSL's all direct gap semiconductors and in particular III-VI alloys based on Zn, Cd, Mn, Mg, Hg, S, Se and Te, where the doping and contact technologies are poorly or not mastered. These technologies become all the more problematical as the gap of the materials increases and therefore the emission wavelength is short. By their very design MSL's lead to the disappearance of these technological problems.
The possibility of using all direct gap semiconductors for MSL's gives accessibility to the wavelength range between the blue and the medium infrared. In particular, MSL's emitting in the blue-green make it possible to satisfy existing needs for all applications concerning optical recording. This range is not at present covered by injection laser diodes, which are the only commercially available compact lasers. Research is at present taking place for producing laser diodes emitting in the blue-green, either from II-VI semiconductors with the difficulties referred to hereinbefore, or using III-V laser diodes emitting in the infrared by frequency doubling or similar non-linear effects.
In the MSL known from FR-A-2 661 566, the focussing of the electrons on the active medium is essentially attributed to the conical or pyramidal shape of the anode. Moreover, use is made of strip focussing means for the electron beam. These means are constituted by two trapezoidal metal screens located on either side of the microdot source and close to the latter.
Focussing solely by the specific shape of the anode is not satisfactory, because it is not possible to control the dimensions or the shape of the focussing spots. The use of two metal screens only makes it possible to control one of the dimensions of the focussing strip.
Although imperfect focussing does not prevent the MSL from functioning, it requires a high operating current in order to achieve the laser threshold current density, which can cause thermal problems and a high stressing of the microdots, making them more vulnerable to breakdowns. In addition, imperfect focussing leads to the loss of part of the electronic pumping beam outside the useful area, which contributes to a reduction in laser efficiency.
Moreover, screens on either side of the source and close to the latter do not screen it from the anode, which is raised to a high voltage. Thus, the microdot cathodes are subject to the effect of the high voltage, that is the source is in a strong electric field zone, which could contribute to an unsatisfactory operation of the source, or could or could shorten its life. The problems referred to hereinbefore lead to a reduction in the performance characteristics and reliability of the known MSL.